The present invention relates to the containment of high temperature, elevated pressure, corrosive reactions, such as supercritical water reactions and supercritical water oxidation reactions.
As used herein, "supercritical water reaction" refers to the broad class of chemical reactions occurring in a mixture containing a substantial portion of water at conditions near or above the critical point of pure water (the critical point of pure water is at 374.2.degree. C. and 217.6 atm). Such reactions are unique in that the properties of water at these elevated temperatures and pressures are much different than at ambient conditions. Supercritical and near-supercritical water possess unique solution, catalytic and dielectric properties and can be highly corrosive. Salts tend to precipitate out of solution, while the water itself tends to act like a strong organic solvent as well as a catalyst for many organic degradation reactions.
"Supercritical water oxidation reaction" refers to a supercritical water reaction in which oxidant (e.g., H.sub.2 O.sub.2, O.sub.2, air) is added which reacts with an oxidizable substance (e.g., an organic) contained in the feed mixture.
Efficient containment of such reactions has become a major concern in commercialization of supercritical water processes due to the corrosive conditions, and the high pressures and high temperatures often required for optimum reaction pathways/kinetics. A current problem with existing commercial supercritical water oxidation reactor designs is that they all require materials which will, over a substantial period of time, withstand: (1) the high pressures of the reaction (greater than about 217 atm, or 3198 psi, or 21,980 KPa), (2) the high temperatures of the reaction (greater than about 374.degree. C., often in excess of about 450.degree. C.), and (3) the corrosive conditions that may occur. No known materials exist which will handle all of these conditions simultaneously.
Metal alloys tend to embrittle (de-anneal), as well as experience creep, when exposed to high temperatures such as those encountered in supercritical water oxidation reactions. This is especially the case for corrosion-resistant metal alloys such as nickel/chromium/iron blends, which embrittle near 500.degree. C. This fact, coupled with the likelihood of corrosion-induced pitting or crazing on the surface of the metal alloy due to the corrosive nature of some supercritical water oxidation reactions, demands that an alternative material be used to contain the high pressure of the supercritical water oxidation reaction.
Certain ceramics and glasses are very resistant to corrosion, but do not possess the mechanical strength to contain the high pressures typical of supercritical water oxidation reactions. Some exotic metals and metal alloys are also corrosion-resistant, but may embrittle and/or creep at high temperatures under strain, or be cost-prohibitive to use on a commercial basis.
References exist in the literature regarding attempts to contain the high temperatures and/or pressures and corrosive natures of certain reactions, e.g., U.S. Pat. Nos. 5,094,753 and 5,132,014 Allington et. al.; U.S. Pat. Nos. 5,160,624 and 5,198,197 Clay et. al.; and U.S. Pat. No. 5,173,188 Winter et. al., teach the incorporation of a removable extraction cartridge used for supercritical fluid extraction. The removable cartridge has an insignificant pressure difference between its inside and outside walls, so that it need not have the strength to withstand significant pressures and can be made out of, e.g., molded plastic for disposable use. The extraction vessel is installed in a heated high pressure vessel. However, the extraction vessel would not effectively contain a high pressure, high temperature, corrosive reaction since, even if the cartridge was made of a corrosion-resistant, temperature resistant material, which is not taught, the same high temperature would be experienced by the entire apparatus, both inside and outside walls. Since the outside walls would be metal, embrittlement, loss of ductility and/or creep would eventually lead to failure of the pressure-containment vessel.
Battelle Pacific Northwest Laboratories (Richland, Wash.) has disclosed a reactor which "uses a thin insert of a corrosion-resistant metal, such as titanium or zirconium, that fits close to the wall of a carbon-steel pressure vessel". The space between the two is filled with a commercial high-temperature heat transfer fluid. The insert is designed so that it can expand toward the pressure vessel outer wall when pressurized. The heat transfer fluid balances the pressure (as described in Chemical Engineering Magazine, December, 1992, page 17). This concept is similar to Allington et. al. in that an outer vessel contains the high pressure while the inner vessel does not experience a large pressure drop across its walls. However, neither Allington et. al. nor Battelle's publication addresses the failure of the pressure-containing vessel when exposed to extended high temperatures such as those of supercritical water reactions. Rather, Battelle's publication teaches transfer of heat from the inner to the outer vessel using a heat-transfer fluid. This type of reactor has the following disadvantages: when the outer carbon-steel vessel is exposed to high temperatures, e.g., in the range of about 400-700.degree. C., it will lose its ductility and may no longer be able to safely provide sufficient strength to contain the pressure. Its effective life is shortened by being brought to high temperatures.
Swift et. al. in U.S. Pat. No. 4,670,404, teaches of using a thin-walled cylindrical batch reactor which is thermally insulated from the walls of a surrounding containment unit, as a pilot apparatus to design full-scale processes and emergency pressure-relief systems. However, Swift et. al. do not address or solve the problem of a potentially corrosive reaction, nor do they address the material concerns associated with an extended high-pressure, high temperature reaction. Rather, their focus is solely to design an emergency relief system which will operate regardless of whether a liquid or gas is discharged from the reactor containing a runaway exothermic reaction. No specific mention is made of containing a high pressure, high temperature, corrosive reaction.
Binning et. al., in U.S. Pat. 4,721,575 and 4,869,833, teach a tubular plug-flow wet-oxidation reactor in which walls are exposed to potentially large pressure drops, while being immersed in a liquid heat-transfer fluid contained in a containment vessel. No solutions were disclosed for containing a high pressure, high temperature, corrosive fluid within the reactor for an extended period. Rather, Binning et. al. focused on improved mixing inside the reactor due to its curved shape.
In U.S. Pat. No. 5,100,560, Huang et. al. teaches a supercritical water oxidation reactor which serves to remove precipitates from the reaction zone as they are formed, but Huang has in no way addressed the issue of high temperature, high pressure, corrosive conditions as in a supercritical water environment.
In U.S. Pat. No. 4,792,408, Titmas et. al. teaches an underground deep-well injection reactor but Titmas et. al. have in no way addressed the issue of high temperature, high pressure, corrosive conditions as in a supercritical water environment.
Significantly, none of these references in any way discloses or suggests a means to contain high temperature, high pressure, corrosive reactions such as supercritical water reactions in an effective, economical and reliable way.
It is an object of the present invention to provide a "reactor within a vessel," in which the "inner" reactor contains the supercritical water oxidation reactants and products, and is made of a material which is resistant to corrosion and can withstand high temperatures, while the "outer" vessel contains the high pressure at a temperature substantially lower than the "inner" reactor.
The reactor of this invention withstands both the operating conditions and corrosive nature of such reactions in a way that is efficient and adaptable to commercial operations.